For example, in-vehicle media playback devices include a device which has a playback system for front seats and a playback system for rear seats, and which can independently play back media data about musical pieces or the like by using each playback system. As examples in which this device is used, there are a case in which the device plays back musical pieces stored in a single audio CD independently by using the playback system for front seats and the playback system for rear seats, and a case in which the device allows each of the playback system for front seats and the playback system for rear seats to independently select musical pieces from musical pieces stored in a hard disk drive (HDD) device to play back them, respectively.
When a user commands each playback system of the media playback device to play back a musical piece, the playback system stars playing back the musical piece while displaying musical piece information about the musical piece on a display unit such as a display. In many cases, each playback system has also an HMI (Human Machine Interface) function of categorizing all the music names of playbackable musical pieces by artist name and genre name, and displaying them on the screen, and playing back one or more musical pieces which the user has selected on the basis of this display screen, and a search function of, when a music name is inputted by the user, searching for the musical piece having the music name, and playing back the musical piece.
As a conventional technology aimed at increasing the efficiency of the search function of searching for a musical piece, there is a music information distribution device disclosed by patent reference 1, for example. A music information database in which TOC (Table Of Contents) information and musical piece information are stored while they are brought into correspondence with each other and a user database used for determining each user's taste in music are disposed in this music information distribution device. When receiving TOC information, this music information distribution device searches through the music information database for musical piece information associated with the received TOC information, and determines whether or not the data volume of the extracted musical piece information is larger than a maximum transmission capacity.
When determining that the data volume of the musical piece information is smaller than the maximum transmission capacity, the music information distribution device determines that the music information distribution device can afford to transmit other musical piece information or the like, and acquires the user's history information from the user database. The music information distribution device increases the efficiency of the search process of searching for a musical piece by searching for and distributing other musical piece information or the like corresponding to an artist name which the music information distribution device has extracted from the music information database through this search by using this history information. However, the conventional in-vehicle media playback device has the following problems when making each of the plurality of playback systems operate independently.
First, a playback system which is connected to a musical piece information display unit via a communication path having a slow data transfer rate and a limited data volume for each transfer takes a remarkably long time to display musical piece information after the musical piece is specified by the user as compared with other playback systems. A problem with a playback system including a communication path having such a slow data transfer rate as above is that the display's response to the user's operation is bad.
Furthermore, in each playback system, musical piece information which is searched for from the musical piece information database is read into a buffer of a processor which controls the musical piece searching process. This reading process requires much processing time, and the number of times that the playback system performs the reading process also increases dependently upon the contents of the musical piece information. More specifically, various resources, such as a CPU resource of the processor and a resource of a bus, are occupied for a long time while musical piece information is searched for, or a situation in which the execution of another process is blocked and hence the completion of the process is delayed occurs. Therefore, there is a possibility that the response of the whole system to the user's operation gets worse.
For example, in a case in which a conventional in-vehicle media playback system has two playback systems and one of them includes a path having a slower data transfer rate than that of the other playback system, when specification of a musical piece from each of these playback systems occurs at random, the response of the operation of the other playback system also gets worse because of processes carried out by the playback system including the path having a slow data transfer rate.
When a congestion occurs in the two playback systems including paths having different data transfer rates in the conventional in-vehicle media playback system, for example, there is a possibility that in either of various operations an increase in the processing load of the whole system and a delay of the process occur, and the system goes beyond the limit of the response ability to respond to the user's operation.
FIG. 6 is a block diagram showing the structure of a conventional, typical in-vehicle media playback device, and this device has two playback systems each for playing back musical piece data. In FIG. 6, the in-vehicle media playback device 100 has a playback system (system 1) which is controlled by a processor 1, and a playback system (system 2) which is controlled by a processor 2, and a searching operation of searching for a musical piece which is requested to be played back by each of the systems 1 and 2 is controlled by a processor 0.
Each of control units 101a and 101b is a component for performing control computations to implement the HMI of the in-vehicle media playback device 100. The processor 1 executes a control program for the system 1 to embody the control unit 101a which performs processing arithmetic of the HMI for the system 1 as one functional configuration unit of the processor 1. Similarly, the processor 2 executes a control program for the system 2 to embody the control unit 101b which performs processing arithmetic of the HMI for the system 2 as one functional configuration unit of the processor 2. Each of the control units 101a and 101b also carries out a process of transferring data between the control unit and the processor 0.
A musical piece information searching unit 102 is a component for searching for musical piece information through a musical piece DB storage unit 104 in response to a search request to search for the musical piece from each of the control units 101a and 101b to acquire the musical piece information, and then sending the musical piece information to the control unit which is the source of the request.
The musical piece information searching unit 102 can also be constructed in such a way as to carry out a process of sorting a plurality of musical piece information which are search results. The processor 0 executes a search control program to embody the musical piece information searching unit 102 as one functional configuration unit of the processor 0.
Musical piece playback units 103a and 103b are components for playing back (decoding) musical piece data read from a musical piece data storage unit 105 according to the musical piece data storage addresses of the musical piece data which are the playback objects specified by the control units 101a and 101b, respectively. The processor 0 also executes a playback control program to embody each of the musical piece playback units 103a and 103b as one functional configuration unit of the processor 0. A musical piece data storage address is a storage address of musical piece data in the musical piece data storage unit 105.
In the musical piece DB storage unit 104, a musical piece database (referred to as a musical piece DB from here on) for storing pieces of musical piece information is constructed. In the musical piece DB, pieces of musical piece information are registered systematically. For example, musical piece information is stored in each record of the musical piece DB storage unit 104.
Each piece of musical piece information consists of a plurality of pieces of item information, such as “music name” information, “artist name” information and “genre name” information which are attributes of the musical piece data, and a musical piece data storage address showing the storing position of the musical piece data in the musical piece data storage unit 105. Each of pieces of item information showing the attributes of a musical piece, such as “music name” information, “artist name” information and “genre name” information, is implemented by character string information.
The musical piece data storage unit 105 stores musical piece data. For example, the musical piece data storage unit is constructed on a storage area of a storage unit, such as a CD, a DVD, an HDD, or a USB (Universal Serial Bus) memory. The musical piece DB storage unit 104 and the musical piece data storage unit 105 can be constructed in the same storage medium, or can be constructed in different storage media, respectively.
Each of musical piece information display units 106a and 106b is a component for displaying musical piece information currently being played back, playbackable musical pieces, etc., and they are installed as a display unit disposed in the in-vehicle media playback device 100. The display processes performed by the musical piece information display units 106a and 106b are controlled by the control units 101a and 101b, respectively. Operation units 107a and 107b are components each for accepting an operational input from outside the in-vehicle media playback device, and output pieces of information inputted thereto through operational inputs to the control units 101a and 101b, respectively. A user performs an operation of specifying a musical piece which he or she desires the in-vehicle media playback device to play back, forwarding to the next track, starting or stopping a playback, or doing the like by using one of the operation units 107a and 107b. Sound output units 108a and 108b are components for generating audio outputs of analog signals of musical piece data whose signal levels are amplified by amplifiers 110a and 110b, respectively. Each of the sound output units is installed as speakers or a headphone.
A vehicle-mounted network 111 is a communication network which connects among the processor 2, the musical piece information display unit 106b, and the operation units 107b, and is a low-speed data transfer path which operates in a CAN (Controller Area Network) or the like. More specifically, the vehicle-mounted network 111 is a path having a slower data transfer rate than either of data transfer paths for connecting between the control units 101a and 101b and the musical piece information searching unit 102, respectively, and a data transfer path for connecting between the network interface of the control unit 101b and the vehicle-mounted network 111.
Furthermore, because the vehicle-mounted network 111 has a slower data transfer rate, a transfer of a lot of data using a single packet (each unit for data transfer) increases the time period during which any other data transfer in the vehicle-mounted network 111 is behind schedule, and there is a possibility that the transfer interferes with the operation of each equipment connected to the vehicle-mounted network 111. To solve this problem, in the vehicle-mounted network 111, an upper limit of the data volume which can be transferred by using a single packet is determined in consideration of the data transfer rate to prevent the above-mentioned failure from occurring, and only an individual transfer of each item information of musical piece information by using a single packet is enabled.
In the system 2, even if high-speed data transfer paths are applied as the path for connecting between the processor 2 and the processor 0 (the control unit 101b and the musical piece information searching unit 102) and the path for connecting between the vehicle-mounted network 111 and the processor 2 (the vehicle-mounted network 111 and the control unit 101b), the efficiency of the data transfer resource gets worse because any data transfer via the vehicle-mounted network 111 is delayed.
Therefore, low-speed data transfer paths are used as the path for connecting between the processor 2 and the processor 0 and the path for connecting between the vehicle-mounted network 111 and the processor 2 in such a way that data transfer operations via the paths are compatible with a data transfer operation via the vehicle-mounted network 111. More specifically, an upper limit of the data volume which can be transferred by using a single packet is determined for each of both the above-mentioned paths, like that determined for the vehicle-mounted network 111, and only an individual transfer of each item information of musical piece information by using a single packet is enabled.
Next, the operation of the conventional in-vehicle media playback device will be explained.
Hereafter, a case in which the musical piece data storage unit 105 stores musical piece data for each track number, and a playback of musical piece data of the same track number “3” is commanded in both the system 1 and the system 2 will be shown as an example.
First, the playback operation of the system 1 will be explained.
FIG. 7 is a timing chart showing the musical piece playback operation of the system 1 in the conventional in-vehicle media playback device. In FIG. 7, the user specifies the track number “3” by using the operation unit 107a (1). Information about the operation of specifying the track number “3” is outputted from the operation unit 107a to the control unit 101a. 
When receiving the above-mentioned operation information, the control unit 101a makes a request of the musical piece information searching unit 102 for a batch transfer of the musical piece information (2). In the batch transfer, all the pieces of item information (“music name” information, “artist name” information, “genre name” information, and “musical piece data storage address”) of the musical piece information are collected into contiguous data, and this data are transferred in a batch as single transfer unit data (a single packet). A “packet” means a unit which occupies a data transfer resource. Furthermore, whether or not the data can be transferred in a batch depends on the data volume on the data transfer path which can be transferred via a single packet.
The control unit 101a then adds identification data about its own identification to the data showing the above-mentioned request. As a result, the musical piece information searching unit 102 can identify that the source of the request is the control unit 101a. 
The musical piece information searching unit 102 searches through the musical piece DB storage unit 104 according to the track number “3” which has been requested by the control unit 101a. At this time, if the track number “3” matches the musical piece information of a record 3 in the musical piece DB storage unit 104, the musical piece information searching unit 102 accesses the record 3 of the musical piece DB storage unit 104 (3), and then reads the musical piece information of the record 3 and writes this information into a buffer (not shown in FIG. 6) of the processor 0 (4).
Next, the musical piece information searching unit 102 reads the musical piece information of the record 3 from the above-mentioned buffer, and transfers this musical piece information in a batch to the control unit 101a which is the source of the request (5). The control unit 101a extracts the musical piece data storage address among the pieces of item information of the musical piece information received from the musical piece information searching unit 102 and specifies the musical piece data storage address for the musical piece playback unit 103a while issuing a playback start command (6).
According to the playback start command, the musical piece playback unit 103a reads and decodes the corresponding musical piece data from the musical piece data storage unit 105 on the basis of the musical piece data storage address specified by the control unit 101a. The decoded musical piece data are outputted from the musical piece playback unit 103a to a DA conversion unit 109a. The DA conversion unit 109a converts the data inputted from the musical piece playback unit 103a from a digital signal into an analog signal, and then outputs this analog signal to the amplifier 110a. The amplifier 110a amplifies the analog signal of the inputted musical piece data, and outputs the amplified analog signal to the sound output unit 108a. As a result, an audio output of the musical piece data which the user has specified is produced via the sound output unit 108a and a playback of the musical piece data is started.
On the other hand, after issuing the playback start command, the control unit 101a promptly carries out a data transfer of the character string information (e.g., the music name information, the artist name information, and the genre name information) of the musical piece information received from the musical piece information searching unit 102 to the musical piece information display unit 106a (7). The musical piece information display unit 106a displays the character string information of the musical piece information received from the control unit 101a on the display screen. The system 1 can thus carry out a playback and a display of the musical piece information of the musical piece specified by the user nearly simultaneously.
Next, the playback operation of the system 2 will be explained.
FIG. 8 is a timing chart of the musical piece playback of the system 2 in the conventional in-vehicle media playback device. In FIG. 8, a user specifies the track number “3” by using the operation unit 107b (1). Information about the operation of specifying the track number “3” is outputted from the operation unit 107b to the control unit 101b. 
When receiving the above-mentioned operation information, the control unit 101b makes a request of the musical piece information searching unit 102 for individual transfers of the musical piece information (2). In the individual transfers, the pieces of item information (the music name information, the artist name information, the genre name information, and the musical piece data storage address) which construct the musical piece information are transferred via separate packets, respectively. At this time, the control unit 101b makes a request for an individual transfer of the musical piece data storage address, among the pieces of item information of the musical piece information corresponding to the track number “3”, to the musical piece information searching unit 102.
The musical piece information searching unit 102 searches through the musical piece DB storage unit 104 according to the track number “3” which has been requested by the control unit 101b. At this time, if the track number “3” matches the musical piece information of the record 3 in the musical piece DB storage unit 104, the musical piece information searching unit 102 accesses the record 3 of the musical piece DB storage unit 104 (3), and then reads all the pieces of item information of the musical piece information stored in the record 3 and writes all the pieces of item information into the buffer (not shown in FIG. 6) of the processor 0 (4).
Because the path connecting between the control unit 101b and the musical piece information searching unit 102 can be used only for individual transfers, as mentioned above, the musical piece information searching unit 102 reads the musical piece data storage address among the pieces of item information of the musical piece information stored in the above-mentioned buffer, and transfers the musical piece data storage address to the control unit 101b which is the source of the request (5). The control unit 101b specifies the musical piece data storage address received from the musical piece information searching unit 102 for the musical piece playback unit 103b while issuing a playback start command (6).
According to the playback start command, the musical piece playback unit 103b reads and decodes the corresponding musical piece data from the musical piece data storage unit 105 on the basis of the musical piece data storage address specified by the control unit 101b. The decoded musical piece data are outputted from the musical piece playback unit 103b to a DA conversion unit 109b. The DA conversion unit 109b converts the data inputted from the musical piece playback unit 103b from a digital signal into an analog signal, and then outputs this analog signal to the amplifier 110b. The amplifier 110b amplifies the analog signal of the inputted musical piece data, and outputs the amplified analog signal to the sound output unit 108b. As a result, an audio output of the musical piece data which the user has specified is produced via the sound output unit 108b and a playback of the musical piece data is started.
On the other hand, even after the playback is started, the control unit 101b makes a request of the musical piece information searching unit 102 for an individual transfer of the “music name” information of the musical piece information corresponding to the track number “3” (7). The musical piece information searching unit 102 searches through the musical piece DB storage unit 104 according to the track number “3” which has been requested by the control unit 101b. Because the track number “3” matches the musical piece information of the record 3 of the musical piece DB storage unit 104, the musical piece information searching unit 102 accesses the record 3 of the musical piece DB storage unit 104 (8), and reads all the pieces of item information of the musical piece information stored in the record 3 and writes them into the buffer of the processor 0 (9).
The musical piece information searching unit 102 then reads the “music name” information from the pieces of item information of the musical piece information stored in the above-mentioned buffer, and transfers the music name information to the control unit 101b which is the source of the request (10). The control unit 101b transfers the “music name” information received from the musical piece information searching unit 102 to the musical piece information display unit 106b via the vehicle-mounted network 111 (11).
Similarly, the control unit 101b makes a request of the musical piece information searching unit 102 for an individual transfer of the “artist name” information of the musical piece information corresponding to the track number “3” (12). The musical piece information searching unit 102 searches through the musical piece DB storage unit 104 according to the track number “3” which has been requested by the control unit 101b. Because the track number “3” matches the musical piece information of the record 3 of the musical piece DB storage unit 104, the musical piece information searching unit 102 accesses the record 3 of the musical piece DB storage unit 104 (13), and reads all the pieces of item information of the musical piece information stored in the record 3 and writes them into the buffer of the processor 0 (14).
The musical piece information searching unit 102 then reads the “artist name” information from the pieces of item information of the musical piece information stored in the above-mentioned buffer, and transfers the artist name information to the control unit 101b which is the source of the request (15). The control unit 101b transfers the “artist name” information received from the musical piece information searching unit 102 to the musical piece information display unit 106b via the vehicle-mounted network 111 (16).
The musical piece information display unit 106b displays the character string information (the music name information and the artist name information) of the musical piece information which the musical piece information display unit has received from the control unit 101b via the vehicle-mounted network 111 on the screen. When displaying the “genre name”, the control unit 101b makes a request of the musical piece information searching unit 102 for an individual transfer of the “genre name” information of the musical piece information corresponding to the track number “3” (17).
The musical piece information searching unit 102 searches through the musical piece DB storage unit 104 according to the track number “3” which has been requested by the control unit 107b. Because the track number “3” matches the musical piece information of the record 3 of the musical piece DB storage unit 104, the musical piece information searching unit 102 accesses the record 3 of the musical piece DB storage unit 104 (18), and reads all the pieces of item information of the musical piece information stored in the record 3 and writes them into the buffer of the processor 0 (19).
The musical piece information searching unit 102 then reads the “genre name” information from the pieces of item information of the musical piece information stored in the above-mentioned buffer, and transfers the genre name information to the control unit 101b which is the source of the request (20). After that, the control unit 101b transfers the “genre name” information received from the musical piece information searching unit 102 to the musical piece information display unit 106b via the vehicle-mounted network 111 (21). As a result, the “genre name” is displayed on the screen of the musical piece information display unit 106b. 
FIG. 9 is a view showing the details of the processing time required for the system 1 to carry out the processing, and that required for the system 2 to carry out the processing in the conventional in-vehicle media playback device. Each processing time in FIG. 9 shows the time which has elapsed until corresponding musical piece information is displayed after the user has specified a track number. Furthermore, in order to compare the processing time of the system 1 with that of the system 2, the processing times shown in the figure are the ones of the systems 1 and 2 each of which is made to operate singly. Each scale of the elapsed time shown in FIG. 9 is equivalent to the transfer time required to transfer a single packet (transfer unit).
In the system 1, each of the process (1) of allowing the user to specify the track number “3”, the process (2) of making a request for a batch transfer of the musical piece information which is performed by the control unit 101a, and the process (3) of accessing the musical piece DB storage unit 104 in response to the request which is performed by the musical piece information searching unit 102 is completed within a time period which is equal to the transfer time required to transfer a single packet in length. In contrast, the process (4) of reading the musical piece information corresponding to the track number “3” from the musical piece DB storage unit 104, and writing the musical piece information into the buffer, which is performed by the musical piece information searching unit 102, requires a long time. The reason why this process requires a long time is that the process involves physical operations of rotating a storage medium which constructs the musical piece DB storage unit 104, and seeking a desired storage area of the storage medium in addition to the electronic operations of the storage medium.
Because every time when musical piece data are registered as a playback object, musical piece information used for musical piece data search is stored in the musical piece DB, the musical piece DB storage unit 104 has a very large data size as registration of musical piece data is made.
Therefore, a semiconductor memory which does not have a storage capacity sufficient for constructing the musical piece DB is not used as the musical piece DB storage unit 104 while a relatively low cost storage medium capable of storing a large volume of data, such as an HDD, a CD or a DVD, is used as the musical piece DB storage unit 104.
Each of the process (5) of carrying out a batch transfer of the musical piece information corresponding to track number “3” to the control unit 101a, which is performed by the musical piece information searching unit 102, the process (6) of specifying the musical piece data storage address for the musical piece playback unit 103a, and issuing a playback start command, which is performed by the control unit 101a, and the process (7) of transferring the musical piece information to the musical piece information display unit 106a, which is performed by the control unit 101a, is completed within a time period which is equal to the transfer time required to transfer a single packet in length.
Also in the system 2, each of the process (1) of allowing the user to specify track number “3”, the process (2) of making a request for the musical piece data storage address of the musical piece information, which is performed by the control unit 101b, and the process (3) of accessing the musical piece DB storage unit 104 in response to the request, which is performed by the musical piece information searching unit 102, is completed within a time period which is equal to the transfer time required to transfer a single packet in length, like in the case of the system 1.
As mentioned above, in the system 2, only individual transfers can be carried out also in the path connecting between the control unit 101b and the musical piece information searching unit 102. Therefore, the control unit 101b makes a request of the musical piece information searching unit 102 for a transfer of each piece of item information of the musical piece information.
However, because the musical piece information searching unit 102 carries out data reading of each record information of the musical piece DB storage unit 104, the musical piece information searching unit carries out the process (4) of reading all the pieces of item information of the musical piece information and writing them into the buffer even when a request for an individual transfer is made by the control unit 101b. More specifically, the musical piece information searching unit repeatedly carries out the processes (4), (9), (14), and (19) of reading all the pieces of item information of the musical piece information stored in the same record whose number is equal to the number of pieces of item information to be displayed on the screen of the musical piece information display unit 106b, and writing them into the buffer.
Furthermore, when performing a playback process of playing back a musical piece medium, the conventional in-vehicle media playback device can list the music name of a musical piece playbackable the next time in addition to the music name of the musical piece currently being played back. Also in this case, such an increase in the processing load and in the processing time as mentioned above occurs in the system 2.
FIG. 10 is a timing chart of a process of displaying the music names of a plurality of musical piece data in the system 2 of the conventional in-vehicle media playback device. As shown in FIG. 10, every time when the control unit 101b makes a request of the musical piece information searching unit 102 for acquisition of “music name” information stored in each of records 1 to 4, the musical piece information searching unit 102 reads the musical piece information from the musical piece DB storage unit 104 in units of one record. More specifically, the musical piece information searching unit repeatedly carries out a process of reading all the pieces of musical piece information from the musical piece DB storage unit 104 whose number is equal to the number of “music names” to be displayed on the screen of the musical piece information display unit 106b. 
A problem with the conventional in-vehicle media playback device is thus that the processing time of the system 2 becomes remarkably long compared with that of the system 1, and the response of display of musical piece information to the user's operation is very bad.
Furthermore, when a long-time process, such as a process of reading and writing musical piece information into the buffer, is carried out two or more times, various resources including the CPU resource of each processor and the resource of the bus are occupied for a long time, or a situation in which the execution of another process is blocked and the completion of this process is delayed occurs. Therefore, the response of the whole system to the user's operation gets worse.
In addition, when specification of a musical piece from the system 1 or 2 occurs at random, the response of the operation of the system 1 also gets worse due to the processing carried out by the system 2. Conventionally, when congestion occurs in the systems 1 and 2, there is a possibility that any of various operations causes an increase in the processing load of the whole system and a processing delay, and the response ability to respond to the user's operation reaches its limit.
For example, in a case in which the musical piece DB storage unit 104 and the musical piece data storage unit 105 are constructed on the same storage medium, a conflict may occur between the process of reading record information of the musical piece DB and the process of reading musical piece data. In this case, a problem is that a delay occurs in the process of reading musical piece data, and the time required to start a playback of a musical piece after the user has specified this musical piece is lengthened.
In order to solve the above-mentioned problem, there can be considered a method of making the musical piece information searching unit 102 individually read musical piece information in units of not each record, but each information which constructs the musical piece information. However, an advantage of shortening the processing time is not provided for the following reasons.
First, in a storage medium in which the musical piece DB storage unit 104 is constructed, such as an HDD, a CD or a DVD, data reading is carried out per sector. For example, in the case of an HDD in which each sector is 512 bytes in size, physical data reading is carried out in units of one sector, i.e. 512 bytes.
Furthermore, the total data size occupied by one musical piece information (the size of one record) usually falls within one sector. Therefore, even if the musical piece information searching unit tries to read each item information of musical piece information individually, the resultant data volume read from the medium does not differ from that in the case in which the musical piece information is read in a batch.
There can be considered a method of newly disposing a reading unit of reading information in units of a smaller amount of data (e.g. one sector or less) in order to read each item information of musical piece information individually.
However, because physical data reading is indispensable in the above-mentioned storage medium, a significant shortening of the processing time cannot be expected as compared with the case in which musical piece information is read out in a batch even if the above-mentioned reading unit is disposed.
The above-mentioned problem is specific to reading of data from the musical piece DB storage unit 104 and a playback device including a data transfer path like the vehicle-mounted network 111, and simple use of the method of improving the efficiency of the data transfer resources described in the patent reference 1 cannot solve the problem.
The present invention is made in order to solve the above-mentioned problem, and it is therefore an object of the present invention to provide an in-vehicle media playback device which reduces the frequency of access to a musical piece database to reduce the processing load thereon and the operation delay occurring therein, thereby being able to improve the ability of response to a user's operation of specifying a playback object.